Higher operating temperatures are sought to increase the efficiency of internal combustion engines and gas turbine engines. To this end, the surfaces of the engine components subjected to the high temperatures have been provided with thermal barrier coatings to increase their durability under the high temperature operating conditions, which may include cyclic temperature and compression loads.
The thermal barrier coatings must have a low thermal conductivity and adhere to the alloy material of the component. Generally, the surface of the component is provided with a thin metallic bond layer to protect the alloy metal component from the corrosive environment to which it is subjected.
Typical metallic bond layers or coatings are an oxidation resistant alloy such as MCr-AlY where M is iron, cobalt and/or nickel. The metallic layer may, for example, be applied by plasma spray vapor deposition. After the metallic protective bond layer has been applied, a thermal barrier ceramic layer is applied. The thermal barrier or coating layer can be applied by plasma spraying an yttrium partially stabilized zirconium layer onto the surface of the metallic bond layer.
Under the severe environmental conditions in which the engine components operate, spallation may occur in localized regions of the thermal barrier coating during engine operation. The coating has in the past been repaired by completely removing the thermal barrier coating from the component, repairing the metallic bond layer, and then recoating the component.
U.S. Pat. No. 5,723,078 describes a method for repairing a thermal barrier coating in which the damaged area is cleaned to expose the metallic bond coating and then a ceramic vapor layer is applied by a plasma spray technique. This prior art technique requires special equipment for applying the coating and is not easily adaptable to field repair of the components.